Quiz — Chapter 31
Multiple Choice
1. Saccharomyces cerevisiae is best described as: - (a) An obligate anaerobe — it dies in the presence of oxygen - (b) An obligate aerobe — it dies in the absence of oxygen - (c) A facultative anaerobe — it can switch between aerobic respiration and anaerobic fermentation - (d) A bacterium that produces alcohol
2. The pathway by which yeast converts glucose to ethanol and CO₂ is called: - (a) The Krebs cycle - (b) Glycolysis followed by ethanolic fermentation - (c) Photosynthesis - (d) Beta-oxidation
3. How many ATP does ethanolic fermentation produce per glucose, compared to aerobic respiration? - (a) 2 ATP for fermentation, ~36 ATP for aerobic respiration - (b) 36 ATP for both - (c) 2 ATP for both - (d) 18 ATP for fermentation, 2 ATP for aerobic respiration
4. The ethanol produced by yeast in bread dough during fermentation: - (a) Stays in the finished bread, making bread mildly alcoholic - (b) Is converted to CO₂ during baking - (c) Mostly evaporates during the long high-heat bake (boiling point 78°C / 173°F) - (d) Bonds chemically with gluten
5. A mature sourdough starter contains approximately: - (a) 100 yeast cells per gram, no bacteria - (b) Yeast only — that's what "sourdough" means - (c) ~10⁷ yeast cells per gram and ~10⁸–10⁹ lactic acid bacteria per gram - (d) Equal parts yeast and bacteria, mostly molds
6. Which is TRUE about sourdough and celiac disease? - (a) Sourdough is safe for celiac patients - (b) Sourdough partially degrades gluten but is NOT a celiac-safe food - (c) Sourdough has no effect on gluten - (d) Sourdough increases the gluten in bread
7. Active dry yeast typically requires "blooming" in warm water before use because: - (a) The drying process leaves a layer of dead cells whose proteins can interfere with gluten if added dry - (b) The yeast has been killed and must be revived - (c) It's a tradition with no scientific basis - (d) The packet warns against direct dough contact
8. Malting is: - (a) The roasting of barley grains at high heat - (b) The controlled germination of grains, producing amylase enzymes that will later convert starch to sugar - (c) The fermentation of beer wort - (d) The crushing of barley with hot water
9. A pale malt versus a dark roasted malt differs primarily in: - (a) The grain species (different barley) - (b) The kilning intensity — dark malts are kilned at higher temperatures, producing more Maillard products - (c) The amount of water used in malting - (d) The yeast strain used afterward
10. Hops contribute three main things to beer: - (a) Sweetness, color, and protein - (b) α-acids (bitterness), aromatic essential oils, and antimicrobial compounds - (c) Yeast, bacteria, and water - (d) Carbohydrates, lipids, and protein
11. Ales differ from lagers primarily in: - (a) The grain used - (b) The yeast strain (top-fermenting S. cerevisiae vs. bottom-fermenting S. pastorianus) and fermentation temperature (warm vs. cold) - (c) The country of origin - (d) The alcohol content
12. Lager yeast (Saccharomyces pastorianus) is: - (a) The same species as ale yeast - (b) A wild yeast found only in caves - (c) A hybrid species formed from S. cerevisiae and S. eubayanus - (d) A bacterium
13. Chicha de jora is: - (a) A Belgian Trappist beer - (b) A traditional fermented corn beverage with thousands of years of Andean history - (c) A yeast strain - (d) A type of malt
14. Bottle-conditioned beer: - (a) Has had CO₂ added under pressure from a tank - (b) Has had a small amount of priming sugar added before bottling, allowing residual yeast to produce CO₂ in the bottle - (c) Has been pasteurized - (d) Contains no yeast
15. The current scientific consensus on alcohol and health is best summarized as: - (a) Moderate drinking is healthy - (b) Red wine specifically is healthy in moderation - (c) There is no level of alcohol consumption that can be considered safe for health - (d) Alcohol is required for good health
Short Answer
16. In 4–6 sentences, explain why a long, cold fermentation produces more flavorful bread than a quick warm one.
17. A friend asks: "If yeast and beer use the same chemistry, why doesn't bread come out alcoholic?" Answer in 2–3 sentences.
18. Describe the role of lactic acid bacteria in sourdough. What do they produce, and what does that do to the bread?
19. Trace the path from a barley grain to a glass of beer. Name at least four steps.
20. Why does a hefeweizen have prominent banana-clove aromas while a German Pilsner does not? Answer in terms of yeast strain and fermentation temperature.
Answer Key
1. (c) S. cerevisiae is a facultative anaerobe — it does aerobic respiration when oxygen is available and switches to ethanolic fermentation when it isn't. This dual capability is why the same yeast can be used for biomass propagation (aerated tanks) and for fermentation (closed dough or sealed beer fermenter).
2. (b) Glycolysis (10 enzymatic steps converting glucose to 2 pyruvate) followed by two more steps (decarboxylation of pyruvate to acetaldehyde, then reduction to ethanol). The Krebs cycle is the aerobic continuation that yeast skips when oxygen is absent.
3. (a) Aerobic respiration extracts ~36 ATP per glucose; fermentation extracts only 2. This 18-fold difference is why yeast multiplies slowly under fermentation conditions.
4. (c) Ethanol's boiling point is 78°C / 173°F, well below baking temperatures of 200°C+. Most of the ethanol evaporates during the bake. Trace amounts may remain but the bread is not alcoholic.
5. (c) A mature sourdough starter has 10–100× more lactic acid bacteria than yeast cells. Calling it "yeast" alone misses half the microbiology.
6. (b) Sourdough's extended fermentation does break down some gluten, which may help people with mild gluten sensitivity, but the residual gluten is far too high for celiac safety. This distinction is medically important.
7. (a) The high-heat drying of active dry yeast produces a layer of dead cells around live ones; rehydrating in warm water lets these dead cells release their proteins into the water, not into your dough where they would interfere with gluten development. Instant yeast, dried more gently with smaller granules, doesn't have this problem.
8. (b) Malting is controlled germination — the grains are wetted, allowed to start sprouting, and then kilned to stop the process. The germinated grain has produced amylase enzymes ready to convert starches into sugars during mashing.
9. (b) All barley (or other) malts come from the same grain species. The kilning intensity (low for pale, high for dark) determines color and flavor through the Maillard reaction at high temperatures.
10. (b) α-acids (bitter when isomerized in the boil), volatile essential oils (aroma — added late or after fermentation for maximum impact), and antimicrobial properties (helping suppress unwanted microbes during fermentation).
11. (b) Top-fermenting ale strains at warm temperatures (15–25°C / 59–77°F) make ales; bottom-fermenting lager strains at cold temperatures (7–13°C / 45–55°F) make lagers. Warm fermentation produces more esters; cold fermentation produces a cleaner profile.
12. (c) Lager yeast is a hybrid that arose from S. cerevisiae (the ale/bread yeast) crossing with S. eubayanus, a cold-tolerant Patagonian wild yeast. The hybrid is cold-tolerant and a strong fermenter — perfect for cool cave storage.
13. (b) Chicha de jora is a fermented corn beer with thousands of years of Indigenous Andean history (Quechua, Aymara, and other peoples), traditionally made by malting the corn (sprouting and drying) and fermenting with wild yeasts. Some traditional preparations used chewed corn to provide salivary amylase.
14. (b) Bottle conditioning adds a tiny amount of fermentable sugar (or fresh wort) along with the beer, and the residual yeast produces CO₂ inside the sealed bottle. This is the traditional method for English ales, Belgian beers, and most homebrews. Forced carbonation under tank pressure is the alternative used by most large commercial breweries.
15. (c) The 2023 WHO statement and recent systematic reviews converge on the conclusion that no level of alcohol consumption is safe for health. The earlier "moderate drinking is good for you" finding is now understood to have been largely an artifact of methodological problems in observational studies. This is a real reversal in the evidence.
Short Answer Model Responses
16. During fermentation, yeast produces not only the main products (ethanol, CO₂) but also many side products including esters, higher alcohols, and other volatile flavor compounds, as well as the lactic acid and acetic acid produced by any bacteria present. These side products accumulate over time. At cooler temperatures, the yeast's overall growth slows, but its flavor-producing side metabolism slows less in proportion, so each hour of fermentation produces more interesting compounds per unit of leavening. Net effect: a cold, slow rise of 18+ hours produces a deeper, more complex flavor than a warm 2-hour rise, even with the same recipe and the same yeast.
17. Bread's ethanol almost entirely evaporates during the high-heat bake — ethanol's boiling point (78°C / 173°F) is well below oven temperatures of 200°C+. Beer is fermented in a sealed vessel and is not heated to drive off the alcohol; it stays. Same chemistry, different post-fermentation handling.
18. Lactic acid bacteria (LAB), primarily Lactobacillus species, ferment sugars into lactic acid (and sometimes acetic acid), dropping the pH of the dough. The acid contributes the sour flavor of sourdough, suppresses spoilage organisms (so the bread keeps longer), partially degrades phytic acid (slightly improving mineral bioavailability), and partially degrades gluten (relevant for non-celiac gluten sensitivity but NOT for celiac safety). LAB and yeast coexist symbiotically in the starter.
19. (a) Malting: barley grains are soaked, allowed to germinate, and kilned to produce amylase-rich malt with controlled flavor and color. (b) Mashing: malt is crushed and steeped in hot water (~65°C), and amylase converts starch into fermentable sugars, producing sweet wort. (c) Boiling and hopping: wort is boiled with hops, sterilizing it, isomerizing α-acids for bitterness, and adding aromatics. (d) Fermentation: cooled wort is pitched with yeast, which converts sugars to ethanol and CO₂ over days to weeks. (e) Conditioning, carbonation, packaging.
20. Hefeweizen uses an ale yeast strain (a S. cerevisiae strain bred for wheat beer) at warm temperatures (typically 18–22°C / 64–72°F). At those temperatures the strain produces lots of esters — particularly isoamyl acetate (banana) — and the phenolic compound 4-vinyl guaiacol (clove). German Pilsner uses a lager yeast (S. pastorianus) at cold temperatures (8–12°C / 46–54°F). Cold fermentation produces few esters, suppresses 4-vinyl guaiacol, and yields a cleaner profile dominated by the malt and hops, not the yeast.